2.4 Field sampling
Field
surveys were conducted in early August 2020. First, the active burrow
entrance at each plot was estimated by the “plugging tunnels method”,
in which the burrows were plugged for 3 days, and the number of plugs
that were cleared by the plateau pikas to allow access to the meadow
surface were recorded (Guo et al., 2012a; Yu et al., 2017b). The average
number of burrow entrances with cleared plugs in 3 days was considered
the density of active burrow entrances per plot; second, the area of
each bare soil patch in the disturbed plot was measured by the
segmentation method (Han et al., 2011), and then the sum of all bare
soil patches areas in each disturbed plot was calculated as the bare
soil area for that disturbed plot; third, five vegetated quadrats (1 m×1
m) were placed on the vegetated surface approximately 8 m apart along a
W pattern in all plots (disturbed and undisturbed plots), and they were
moved slightly to avoid bare soil patches in disturbed plots if
needed;
fourth, a bare soil patch was selected as a paired bare soil
quadrat
for each vegetated quadrat in the disturbed plot, and the distance
between each paired bare soil quadrat and vegetated quadrat was as short
as possible, less than approximately 1 m. Thus, there were five paired
quadrats, consisting of five vegetated quadrats and five bare soil
quadrats in each disturbed plot, and there were five vegetated quadrats
in each undisturbed plot since this study only focused on bare soil
patches induced by plateau pikas.
In each vegetated quadrat of the disturbed or undisturbed plot, all
vascular plant species were identified, and their number was recorded as
the plant species richness of a community. Then, all plants rooted in a
quadrat were harvested into palatable and unpalatable plants (Pang &
Guo, 2017). Finally, all palatable plant samples were placed into
envelopes and carried back to the laboratory.
Generally, most burrows derived from plateau pika activities were less
than 20 cm depth (Yu et al., 2017b), although a few burrows extended to
depths of 60 cm (Fan et al., 1999). In addition, the majority of the
plant roots in the alpine grasslands of the Qinghai-Tibetan Plateau are
in the top 20 cm of the soil. Therefore, this study collected soil
samples at a depth of 20 cm. The soil samples were collected from
vegetated quadrats and bare soil quadrats for each disturbed plot,
whereas they were collected from vegetated quadrats for each undisturbed
plot. Before collecting the soil samples, plants and litter were cleared
from the soil surface. First, a 5-cm diameter soil auger was used to
collect soil samples, with which soil organic carbon and soil nutrient
concentrations (total nitrogen, phosphorus and potassium) were measured;
second, soil profiles in each quadrat were produced by a spade, and a
stainless-steel cutting ring (the volume was 100 cm3)
was used to collect soil cores to determine soil bulk density and soil
water content. The soil samples for measuring soil bulk density were
quickly packed into aluminum boxes with recorded weight, and each
aluminum box was numbered. The aluminum boxes with fresh soil were
weighed and recorded on the spot, and these aluminum boxes with soil
samples were stored at 4°C and transported to the laboratory.